feat(gateway): 支持强制 Codex CLI 模式并伪装 UA

- Codex CLI 请求仅使用内置 instructions，不再读取 opencode 缓存/回源\n- 新增 gateway.force_codex_cli（环境变量 GATEWAY_FORCE_CODEX_CLI）\n- ForceCodexCLI=true 时转发上游强制 User-Agent=codex_cli_rs/0.0.0\n- 更新 deploy 示例配置

backend/internal/handler/openai_gateway_handler.go

@@ -28,6 +28,7 @@ type OpenAIGatewayHandler struct {
 	errorPassthroughService *service.ErrorPassthroughService
 	concurrencyHelper       *ConcurrencyHelper
 	maxAccountSwitches      int
+	cfg                     *config.Config
 }
 
 // NewOpenAIGatewayHandler creates a new OpenAIGatewayHandler
@@ -54,6 +55,7 @@ func NewOpenAIGatewayHandler(
 		errorPassthroughService: errorPassthroughService,
 		concurrencyHelper:       NewConcurrencyHelper(concurrencyService, SSEPingFormatComment, pingInterval),
 		maxAccountSwitches:      maxAccountSwitches,
+		cfg:                     cfg,
 	}
 }
 
@@ -109,7 +111,8 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 	}
 
 	userAgent := c.GetHeader("User-Agent")
-	if !openai.IsCodexCLIRequest(userAgent) {
+	isCodexCLI := openai.IsCodexCLIRequest(userAgent) || (h.cfg != nil && h.cfg.Gateway.ForceCodexCLI)
+	if !isCodexCLI {
 		existingInstructions, _ := reqBody["instructions"].(string)
 		if strings.TrimSpace(existingInstructions) == "" {
 			if instructions := strings.TrimSpace(service.GetOpenCodeInstructions()); instructions != "" {